Barch BS Q&A Papers 1

7/26/2019 Barch BS Q&A Papers 1

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MAY/JUNE 2013.

PART-A

1. What do you mean by evao!at"on and #ub$"mat"on%

Evao!at"onis the process of a substance in a liquid state

changing to a gaseous state due to an increase in

temperature and/or pressure. Evaporation is a fundamental

part of the water cycle and is constantly occurring

throughout nature. During Evaporation water changes from a

liquid to a gas or vapor.

&ub$"mat"onis the transition of a substance directly from

the solid to the gas phase without passing through an

intermediate liquid phase.

2. What do you mean by #en#"b$e heat and $atent heat%

&en#"b$e heat 'When an object is heated, its temperature rises as heat is

added. The increase in heat is called sensible heat. imilarly,

when heat is removed from an object and its temperaturefalls, the heat removed is also called sensible heat. !eat that

causes a change in temperature in an object is called

sensible heat.

(atent heat

!eat that causes a change of state with no change in

temperature is called latent heat. "ll pure substances in

nature are able to change their state. olids can becomeliquids #ice to water$ and liquids can become gases #water to

vapor$ but changes such as these require the addition or

removal of heat. The heat that causes these changes is

called latent heat.


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3. )e*ne ton !e+!",e!at"on.

The unit of measure used in air conditioning to describe thecooling capacity of a system. %ne ton of cooling is based on

the amount of heat needed to melt one ton of ice in a &'

hour period. %ne ton of cooling is equal to (&,))) *tu/hr or

+)&' cal/hr.

. )!a the T-# d"a,!am o+ a vao!# om!e##"on y$e

and "nd"ate $ea!$y the va!"ou# !oe##e#.

" Temperature-Entropy diagram, or T-s diagram, is used inthermodynamics to visualie changes to temperature and specic

entropy during a thermodynamic heat transfer process or cycle.

Temperature indicates the degree or intensity of heat present in a

substance. Entropy is a measure of the energy available for wor0.

"lmost all 1efrigeration and "ir-conditioning equipment use vapor-

compression cycles cycle to produce cooling. The 2apor-

3ompression 1efrigeration 3ycle is comprised of four steps.

" wor0ing 4uid #often called the refrigerant$ is pushed through the

system and undergoes state changes #from liquid to gas and

bac0$. The latent heat of vaporiation of the refrigerant is used to

transfer large amounts of heat energy, and changes in pressure

are used to control when the refrigerant e5pels or absorbs heat

energy.

(6 3ompression6

7n this stage, the refrigerant enters the compressor as a gas

under low pressure / low temperature. Then the refrigerant is

compressed and the gas leaves the compressor under high

pressure / high temperature.


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&6 3ondensation6

The high pressure, high temperature gas releases heat energy

and condenses inside the 3ondenser. Thus the refrigerant leaves

as a high pressure / high temperature liquid.

+6 E5pansion # Throttling $6

The high pressure / high temperature liquid refrigerant is pushed

through a E5pansion #throttling $ valve, which causes it to e5pand.

"s a result, the refrigerant is now low pressure / low temperature

liquid.

'6 Evaporation

The low pressure / low temperature liquid refrigerant enters the

evaporator. The refrigerant now cools the air or water surrounding

the evaporator. "s it has pic0ed up the heat the liquid is now

converted to gas. The refrigerant leaves the evaporator as a low


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temperature / low pressure gas and is ta0en into the compressor

again, bac0 at the beginning of the cycle.

. What "# meant by hum"d"*at"on and

dehum"d"*at"on%

!umidication 6 The process of increasing the water vapour /

moisture content of air.

De-!umidication 6 The process of decreasing the water vapour /

moisture content of air.

. ("#t don the advanta,e# o+ +an o"$ heat"n, #y#tem#.

7f multiple smaller ones have to be heated then there are

two methods of doing it.

"ll air heating system 6 We can heat air in one common place

and distribute the hot air to various ones by means of air

distribution syste.

"ll water heating system 6 We can heat water in one

common place, then pump the hot water to various ones

through pipes. These pipes will be connected to each 8an coil

unit installed in each one. The fan coil unit consists of lter,

blower, heating coil and controls. The hot water enters the

heating coil and in turn heats the air coming into contact

with coil.

"dvantages 6

Each one can be controlled individually. 7f a particular one

is not utilied then the hot water will not enter the fan coil

and thus the one is not heated. This will turn ensure that


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hot water production is lowerd based on the usage. Thus

power saving is ensured during part load operations.

This is not possible in "ll air heating system. Elaborate

controls li0e variable air volume and variable frequencydrives will have to be incorporated.

"ll water system do not use ducting which occupies

considerable space above false ceiling. They also don9t need

an air !andling :nit 1oom which again occupies space.

7t is easier to control water than air and during part load

operations the response time for water systems is faster

than air system.

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PART-

. 4a5. E6$a"n the o!7"n, o+ E$et!" moto!.

"n electrical motor is an electromechanical device which converts electrical

energy into a mechanical energy. "lmost all the motors used in !2"3

application is 7nduction @otor. The name coming from the fact that the


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current in the moving part is induced, the moving component having no

connection to the source of current.

"n "3 motor has two basic electrical parts6 a AstatorA and a ArotorA as shown

in 8igure . The stator is in the stationary electrical component. 7t consists of a

group of individual electro-magnets arranged in such a way that they form a

hollow cylinder, with one pole of each magnet facing toward the center of

the group.

The term, AstatorA is derived from the word stationary. The stator then is the

stationary part of the motor. The rotor is the rotating electrical component. 7t

also consists of a group of electro-magnets arranged around a cylinder, with

the poles facing toward the stator poles.

The rotor, obviously, is located inside the stator and is mounted on the

motorBs shaft. The term ArotorA is derived from the word rotating. The rotor

then is the rotating part of the motor. The objective of these motor

components is to ma0e the rotor rotate which in turn will rotate the motor

shaft. This rotation will occur because of magnetic phenomenon that unli0e

magnetic poles attract each other and li0e poles repel. 7f we progressively

change the polarity of the stator poles in such a way that their combined

magnetic eld rotates, then the rotor will follow and rotate with the magnetic

eld of the stator.


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48!5

#b$. Write notes on

". A"! 9and$"n, Un"t.

"n "ir-!andling :nit #"!:$, is a device used to condition and circulate air aspart of a !2"3 system. The primary function of an "!: is to transmit

processed air from the air conditioning plant to the conditioned space and

distribute it properly within the conditioned space.

"n "!: is usually a large modular metal bo5 containing various sections

li0e 6

8ilter ection with pre-lters

3ooling 3oil ection with copper tubes and aluminium ns.

*lower section with 3entrifugal fans and @otor.

"ccessories include drain pan, dampers , 28D, tarters, 2ibration

isolators etc

The air is rst passed through lters to remove dust particles and then over

to the cooling coils wherein the air is cooled and de-humidied. The blowers

then convey the pressuried air to the ducts which in turn distributes to the

area to be air-conditioned through grilles and diCusers..


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8


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;"T:1"< D1"8T 3%% T%WE1@E3!";73"< D1"8T 3%% T%WE1

;. 4a5. )"#u## "th neat #7ethe# the "ndo tye and

a7a,ed tye ond"t"one!#.

W


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side are separated from each other by an insulated partition

enclosed inside the window air conditioner assembly.'. Window air conditioners are mainly used for residences, small

oces and shops. "dvantages are easy to install and cheaper.

Disadvantages are no fresh air and not suited for large areas. The

units are also noisy.

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"dvantages are 6

:nits are rugged

@ultiple units can be used for large areas

Easy and fast installation

3an handle long duct runs and higher air quantity requirements

ervices are restricted to the unit room.

Disadvantages 6

@ultiple units for required for larger applications.

8loor space required for locating the units.

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48!5

#b$. Write note on compressors used in refrigerators.


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The compressors used for 1efrigerators are broadly called as G

!ermetically ealed 3ompressors G. 7n this compressor both the

compressor and motor are conned in a single outer welded steel

shell. The motor and compressor are directly coupled on the same

shaft, with the motor inside the refrigeration circuit.

The main advantages are 6

;o refrigerant lea0.

"ll components are factory assembled hence quality can be

maintained.

;o servicing is possible at site hence chances of meddling at

site is eliminated.


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There are two scrolls wherein one of the scrolls is 5ed, while the

other orbits eccentrically without rotating, thereby trapping and

compressing poc0ets of refrigerant gas between the scrolls.

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31%


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"dvantages 6 " DH plant is most ecient from the thermodynamic point of

view since the heat transfer is directly between the refrigerant and air. 8irst

cost is lower. impler to operate and service.

Disadvantages 6 Ducts running from one 4oor to another 4oor occupies huge

4oor space. =art load operations are very poor. The plant and "!: have to be

0ept close as the gas cannot travel more than & @eters.


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:ENTRA( )@ P(ANT :ENTRA( :9


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The dimensions of the area to be air-conditioned.

*uilding orientation.

:sage activity of the area to be air-conditioned.

:sage activity of the adjoining areas.

!eight / *eam !eight. 8resh quantity to be provided based on 2entilation

standards.

>lass area and type of glass in all orientation.

E5posed Wall area and type of construction in all orientation.

;on-e5posed # called as partition wall $ and type of

construction.

7f the 4oor below / ceiling above / roof above is non-

airconditioned then heat from these sources have to beconsidered. The type of contruction also has to be ta0en into

account.

%ccupancy.

%utside air

The ne5t step is to nd the air quantity required from this system

to maintain the inside design conditions.

8or this we need to do the following calculations 6


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tep-( 6 E1! is already 0nown.

tep-& 6 8ind the sum of E1! I E1

;ow nd the ratio of tep-( divided by tep-&. This ratio is

called ensible !eat 8actor F !8.

Then from the tables, based on the 1oom temperature,1oom humidity and !8 we can nd the "pparatus Dew

=oint F "D=.

*ased on E1! , 1oom Temperature and "D= we can nd the

"ir Juantity required in 3ubic 8eet per @inute # 38@ $.

8inally we have to select the air-conditioner which satises

both the 3apacity in T1 and "ir quantity in 38@.

7f only the T1 is ta0en into account and not the 38@ then the

inside conditions cannot be maintained.


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Barch BS Q&A Papers 1

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